Silverlight 5: Set ConstructorArgument on a IMarkupExtension class

Question

I would like to have a MarkupExtension that can be initiated with just one value, much like Binding:

<TextBox Text="{Binding MyPath}"/> instead of <TextBox Text="{Binding Path=MyPath}">

In WPF this is done with the ConstructorArgumentAttribute, but this doesn't exist in Silverlight.
I checked for attributes on the Path property of Binding but have'nt found any attributes applied to it.

Any ideas?